I have a series of utilities to generate Google sitemaps for my whole site. These files are massive, and slow to build. We want to start telling Google these pages are mobile-crawl-able too, by adding them to mobile sitemaps, but the documentation is unclear if I need to specify physically different files for my mobile URLs than for my normal ones.

If this is my current sitemap:

<?xml version="1.0" encoding="UTF-8" ?>
 <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
    <url>
        <loc>http://mobile.example.com/article100.html</loc>
    </url>
</urlset>

Can I simply change it to:

<?xml version="1.0" encoding="UTF-8" ?>
 <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
  xmlns:mobile="http://www.google.com/schemas/sitemap-mobile/1.0">
    <url>
        <loc>http://mobile.example.com/article100.html</loc>
        <mobile:mobile/>
    </url>
</urlset>

Or do I need to create new files with the additional markup, alongside my existing files?